"thong for holding a dog or hound," c.1300, from Old French laisse "hound's leash," from laissier "loosen," from Latin laxare, from laxus "loose" (see lax). Figurative sense attested from early 15c. The meaning "a set of three" is from early 14c., originally in sporting language.

v.

"to attach to or with a leash," 1590s, from leash (n.). Related: Leashed; leashing.